Lecture 7

Etching

Figures of merit• Etch rate: sufficiently high but

not too high, usually 100-1000 Å/min

• Etch rate uniformity: percentage variation of etch rate

• Etch selectivity: etch rate of various materials

• Etch anisotropy (lateral vs. vertical)

1 L

V

RAR

= −

• Material damage produced by etching

• Safety to health and environment

Wet etching

Advantages• highly selective• doesn’t introduce damageDrawbacks• lack of anisotropy• poor process control• excessive particle contamination

etching species reaction products

reaction at the surface

• transport limitation;• bubble formation• resist scumming

Wet etching of SiO2.

• Wet etching in dilute solutions of HF: 6:1; 10:1 and 20:1 (HF:H2O)Typical etch rate for thermal oxide (6:1): 1200 Å/min

2 2 6 26 2SiO HF H SiF H O+ → + +

To avoid pH and concentration change, buffered HF (BHF) is used

4 3NH F NH HF+

Etching rates: BHF 20:1: thermal oxide 300 Å/min

Si3N4 10 Å/min

Wet etching of SiO2.

• Etched groove (with and without steering)

Other wet etching recipes

• Silicon Nitride (if oxidized at high T, oxide should be removed first)– H3PO4 at 140 – 200 °C– 3:10 mixture of 49%HF and 70%HNO3 at 70 °C.

• Aluminium etch (alloys might be difficult!)– 20% v/v acetic acid, 77% H3PO4, 3% HNO3.

Isotropic etching of Silicon• Silicon etching (oxidation + HF)

3 2 6 2 2 2Si HNO H SiF HNO H H O+ + + +

acetic acid is commonly used as a diluent

HF limited

oxidant limited

Wet etching of Si

Etch rate depends on the crystallographic orientation. Highlypacked planes are the slowest to etch

OHOHSiOOHOHSi

HOHeOH

eOHSiOHSi

2222

22

22

22

2)(4)(

444

4)(2

+→+

+→+

+→+

−−+

−−

−+−

Types of etchants:• Alkali hydroxide etchants: KOH, NaOH, CsOH, RbOH• Ammonium hydroxide etchants: Nh4OH, tertramethylammonium hydroxide (TMAH) (CH3)4NOH • Other etchants: hydrazine/water, amine gallate etc.

Anisotropic etching of Silicon

• Crystallographic orientations (Miller indices)

Anisotropic etching of Silicon• Crystalline structure of Silicon

• Diamond like structure

• Marking orientationon Silicon wafers

Anisotropic etching of Silicon• Wet etching of (100) Silicon

Anisotropic etching of Silicon

Anisotropic etching of Silicon• Etching rate for many alkaline solutions is slow in (111)

direction, so anisotropic etch with high aspect ratio can be achieved

• Mixture: (KOH:Isopropyl Alcohol: Water) (23.4 : 13.5 : 63)gives etching rate ratio (100)/(111) 100/1.

• Special purpose wet etch– doping selective etch:

HF/HNO3/CH3COOH 1:3:8 – 15 times higher etch rate for heavily doped silicon (>1019 cm-3) ethylene-diamine-pyrocatehol-water etches lightly doped silicon without attacking heavily doped

– Defect selective etchrate ratio (100)/(111) 100/1.

Anisotropic etching of Silicon• Chracteristics of different etchants

Wet etching of other orientation of Silicon

Chemical Mechanical Polishing (CMP)

containing 12-30% of SiO2 particles, KOH and NH4OH, pH~10 to keep silica negatively charged

34 /(2 )S pR P K Eφ=

Resulting smoothness:

• used to achieve global planarization: globally flat surface, free of scratches and contamination

Plasma etching

Advantages:• easy start and stop• less sensitive to small changes in temperature• large variety of chemistry, anisotropic etch with tunable

anisotropy and selectivity can be achieved

Plasma etching• DC plasma

(glow discharge)

Plasma etching• RF plasma

High-Pressure Plasma Etching• Historically, the first plasma systems• high pressure, typically 500mTorr• mean free path shorter than the chamber size, low

ion energy• etch process depends primarily on the chemistry

Example: CF4 plasma

17 /C F Si Si Si F kJ mol∨ + ∨ = − +105 kJ/mol 42 kJ/mol 130 kJ/mol

net positive energy (from collision with high-energy electrons) is required for the reaction

High-Pressure Plasma Etching• Mechanisms for Si etching in

CF4 plasma:

• Rate can be estimated by flux of reactive species (F2, CFx, x=1,2,3)

2

2nn RTJ

Mπ=

High-Pressure Plasma EtchingSiO2 etching:

5 /F F Si O Si F Kcal mol∨ + ∨ = − + −

– SiO2 etching is more aggressive; selectivity Si/SiO2 is 50:1 at room T, 100:1 at -30 ºC

– high concentration fluorine is preferable for high Si/SiO2 selectivity

– preferred species CF4, C2F6, SF6

– small addition of oxygen improve both etch rates, large addition removes selectivity

High-Pressure Plasma Etching

• Anisotropic etchfluorocarbon fil passivating the wall

Formation of hydrocarbon film can be encouraged by addition of hydrogen (H2, CHF3 etc.)

High-Pressure Plasma Etching

• Anisotropic etch

High-Pressure Plasma Etching

• Loading effect: etch rate in may plasma etches decreases with increase in the area of exposed film

0

1RRkA

=+

etching rate for an empty chamber

area of the exposed film

• End-point detection:– laser interferometry– mass spectrometry analysis of the gas composition

Ion Milling• Accelerated ions of noble gases are used• purely mechanical etching• high directionality• applicable to wide range of materials

Kaufman source

• independent control over ion and ion energy

• beam divergences 5° - 7°• large ion currents over large

area

3/ 2

max 2t

g

Vqj Km I

≈

Ion Milling• typical problem during ion milling

Ion Milling

• Chemically assisted Ion-beam milling (CAIBE): reactive species are added to the beam (e.g. O2)

• Ion-assisted chemical etch

Reactive Ion Etching (RIE)• in RIE the wafer rests on the powered electrode that increases

bombardment energy. • the pressure is lower to improve plasma contact with the wafer

Typically50mTorr5kW/cm2.

Reactive Ion Etching (RIE)• Si etching in Cl plasma

Reactive Ion Etching (RIE)

• Sidewall passivation in HCl/O2/BCl3 plasma

Reactive Ion Etching (RIE)

• Problem during RIE– residual damage in the

substrate after etch due to ion bombardment.

– chemical contamination (due to polymerization; metallic impurities due to sputtering of chamber, electrodes etc, )

O2 plasma cleaning followed by wet acid cleaning and H2 plasma treatment

carbon can penetrate as deep as 300Å (forming Si-C bonds), hydrogen as deep as several microns. Solution: Cleaning + Annealing

Reactive Ion Etching (RIE)

• Problem during RIE– residual damage in the

substrate after etch due to ion bombardment.

– chemical contamination (due to polymerization; metallic impurities due to sputtering of chamber, electrodes etc, )

O2 plasma cleaning followed by wet acid cleaning and H2 plasma treatment

carbon can penetrate as deep as 300Å (forming Si-C bonds), hydrogen as deep as several microns. Solution: Cleaning + Annealing

Summary dry etching

Lift Off

• Process sequence

Lift Off• resist profiles after different treatment

Problem:

• You have to machine an orifice 10 x 10 um through a Si wafer(520um thick). What should be the size of the mask on theback side? How long will it take? You are going to use KOH solution with etching rates 5nm/min in {111} planes and 2um/min for {100} planes.

• 11.2• 11.3